The field of the disclosure relates generally to gasification, such as gasification used in Integrated Gasification Combined Cycle (IGCC) power generation systems, and more specifically to systems and methods for supplying high moisture content, solid, carbonaceous fuels to gasifiers, and methods of start up for such systems.
At least one known IGCC plant feeds a water-based slurry of bituminous coal to a refractory-lined, entrained flow gasifier to generate the fuel gas used in power generation. Such a slurry feed system may provide an economical and reliable option for feeding higher rank coals, such as bituminous and anthracite coals, to the gasifier. However, such a system is less attractive for lower rank coals, such as sub-bituminous coals, because of the difficulty surrounding the production of low rank coal slurries with a solids concentration and energy content high enough for efficient power production.
Inherent moisture is water trapped in the pores of the coal and therefore such moisture may not be available for making the coal slurry. Low rank coals have a relatively higher inherent moisture content (e.g. 22-30 wt %) compared to high rank coals (e.g. <10 wt %). In known IGCC systems, the production of coal-water slurry is a physical process that includes suspending the coal particles in water to facilitate enabling the coal particles to freely move past one another, i.e. enabling slurry flow within the IGCC system. More specifically, in some known IGCC systems, water may be added in an amount sufficient to produce a slurry with a viscosity no higher than about 700 to 1000 Centipoise to enable the slurries to be screened, pumped and sprayed by the feed injectors. Coals with higher inherent moisture content naturally produce slurries with higher total water content. For example, coals with relatively higher inherent moisture content produce slurries with a lower solids content, i.e. lower energy content per unit volume of slurry. While water may be added to particulate sub-bituminous coal to produce a pumpable slurry, the energy content of the resulting dilute slurry may not reach an energy level capable of sustaining an efficient gasification operation.
In some known IGCC systems, the quantity of water needed to make a pumpable slurry far exceeds the amount of water needed for the reactions. Although some of the water does react with the coal and convert the coal to syngas, most of this excess slurry water passes through the gasifier, consuming some of the thermal energy in the reactor as the water heats up to reaction temperature, and then degrading that thermal energy produced in the gasifier to lower temperature levels as the product gas is cooled in downstream equipment. The extra energy required for heating the excess water to gasifier reaction temperature comes at the expense of burning some of the CO and H2 in the product syngas to CO2 and H2O. This requires additional oxygen to be fed to the gasifier, which decreases efficiency and increases capital cost. Also, by converting some of the CO and H2 in the product syngas to CO2 and H2O in order to heat up the excess water, the amount of CO and H2 produced per unit of coal gasified decreases. Therefore, in order to fuel the power block with a fixed amount of CO and H2, the syngas components with fuel value, a larger amount of coal must be gasified when feeding a coal slurry compared with feeding coal in a much drier state. This increased coal requirement both decreases the plant efficiency and increases its capital cost.
Some known combustion turbines must burn a fixed amount of carbon monoxide and hydrogen to achieve their maximum rated power production. To produce the required amount of CO and H2, a plant feeding a dilute slurry of sub bituminous coal must gasify significantly more coal than a plant feeding a slurry of bituminous coal. Such a sub-bituminous coal plant may be both less efficient and more costly to construct and operate.
Some known IGCC systems feed high moisture content coal to gasifiers using a system known as a dry feed system to overcome the difficulty of producing a high energy content slurry and to avoid the negative impact on overall plant efficiency. In such a dry feed system, lower rank coals may be dried to remove two-thirds, or more, of the inherent moisture present in the coal. The deep drying facilitates improving the flow characteristics of the dried solids in the dry feed system equipment as well as improving the overall efficiency of the gasifier. For instance, high levels of drying are often needed to help reduce the potential consolidation and subsequent flow problems that can result during pressurization of higher moisture content solids in a lock hopper. However, drying the coal may consume a large amount of energy, which reduces the overall power production of the plant as a result. In addition, the dry feed system equipment, which may include a compressor, lock hoppers, lock hopper valves, drying equipment and additional storage capacity, results in a relatively expensive system when compared with slurry-based systems. Furthermore, such systems are limited to relatively modest pressures, on the order of 400 psig or less, because the consumption of gas used for lock hopper pressurization and particle fluidization increases dramatically as system pressures increase.
In some known IGCC systems with slurry fed gasifiers, a two-step process may be used for gasifier startup that includes establishing steady flows of all feeds in bypass and/or startup conduits not connected to the gasifier, and redirecting the flows into feed conduits connected to the gasifier feed injector according to a prescribed sequence. Pre-establishing the flows to the gasifier using this two-step process ensures that the correct fuel-oxidant mixture is delivered to the feed injector which, in turn, assures a substantially safe and reliable startup. The startup slurry flow in a slurry feed system is established in a circulation loop that returns to the original slurry storage tank, and the startup oxygen flow may be vented to atmosphere through a silencer. Upon startup, the slurry and oxygen flows are diverted into the gasifier so that the oxygen reaches the feed injector a short time after the slurry. The thermal energy stored in the preheated gasifier refractory brick ignites the reaction mixture and the gasification reactions begin. In contrast, some known IGCC system use a moist feed (or dry feed) system to feed a gasifier. The fuel in a moist feed system is not a storable material like coal-water slurry. Instead, the fuel is manufactured, or “assembled”, prior to introduction into the gasifier by mixing solid fuel particles into a flowing stream of carrier gas. If this mixture is channeled through a conduit but not consumed in the gasifier, such a stream may not be stored for later use, and the carrier gas—fuel particle mixture must be “disassembled” so that the solid fuel particles can be returned to storage for later use.
Some known moist feed IGCC systems use a two-step startup method in which nitrogen from the air separation unit may be used as a carrier gas during startup operations. During the first step of the startup method when steady flows of all gasifier feeds are established in bypass and/or startup conduits not connected to the gasifier, the nitrogen carrier gas-fuel particle mixture is returned to the original particulate fuel storage bin via one or more gas-solids separation devices. The gas-fuel solids separation devices facilitate removing a high percentage of the nitrogen from the solid particles before returning the solid particles to a storage bin. The nitrogen is subsequently cleaned to remove moisture and very fine particles so that the nitrogen may be reused in the feed system as carrier gas and/or inert blanketing gas. Some of the nitrogen may be vented to the atmosphere as a purge gas which excludes air from the feed system and facilitates maintaining an inert environment within the feed system. Because nitrogen is used throughout the moist feed system, it may not be necessary for the gas-fuel solids separation devices to be 100% efficient in removing nitrogen from the solid fuel particles. Thus, it is generally acceptable to return the unconsumed solid fuel particles to their original storage bin for later reuse.
However, in the case where nitrogen may be unavailable for use as a carrier gas, it may be necessary to use a different carrier gas during startup, such as for example, a process-derived gas such as a sour CO2-rich gas and/or syngas. In this case, solid fuel particles may not be returned to their original storage bin. The residual process-gas trapped in the pores of the solid fuel particles, along with any process gas entrained by those particles as they return to the storage bin, may contaminate the feed system with a gas that ultimately cannot be vented directly to the atmosphere. In order to use a process-derived gas, i.e. a “non-ventable” gas, as a startup carrier gas a different startup method and process configuration is needed.